Production of a surface area with metallic conductivity on oxidised al-mg alloys

ABSTRACT

The invention relates to a method for the remetallisation of surface areas on Al— and/or Mg alloys by means of laser bombardment and then by sealing using a free-flowing and electroconductive substance.

[0001] The present invention relates to remetallization of oxidized and/or phosphatized surfaces on Al and Mg alloys. Since Al and Mg never occur as pure elements in technical reality, the present invention generally speaks of alloys. This is meant to include also alloys in which one of both metals appears almost exclusively.

[0002] Al, Mg, and in general also their alloys, are easily oxidized materials on which oxide layers appear rapidly. The term “oxidation” in its chemical sense shall include each reaction in which the oxidation state or number is increased by a loss of valence electrons, especially a reaction with oxygen. This can appear on oxidized surfaces that have been produced accidentally by contact to air or other oxidizing substances. The invention, however, is also related to the remetallization of surfaces oxidized and/or phosphatized intentionally for technical reasons, which are known to the skilled person from various applications.

[0003] It is often intended to produce electrically conductive portions on such surfaces after oxidation/phosphatizing, especially for producing an electrical contact to a bulk alloy thereunder. The invention relates also to the case in which the oxide/phosphate layer is removed down to the metal alloy thereunder for other reasons than electrical reasons, in which, however, an actually metallically conductive surface is produced.

[0004] The invention has the object to produce metallically well conductive surface portions on oxidized and/or phosphatized surfaces of Al and/or Mg alloys.

[0005] According to the invention, this object is achieved by a method for producing a metallically conductive surface portion in a surface of an alloy being oxidized and/or phosphatized by anodisation, said alloy comprising Al and/or Mg, in which method said surface portion is bombarded with a laser in order to remove the oxide/phosphate layer and said surface portion is covered thereafter with a metallically conductive fluid substance.

[0006] The terms “oxidized surface” and “phosphatized surface” do not only refer to surfaces having a coverage of pure oxides or phosphates of the respective alloy. Also other substances can be included that have been produced in preceding intentional or accidental reactions. Further, the surface can be covered with other layers, additionally, that have to be removed also. Especially, the invention refers to surfaces on which mixtures of oxides and phosphates are present, e. g. to surfaces with phosphate layers having incorporated oxides.

[0007] First it has been concluded that it is a prerequisite for a metallically conductive surface with correspondingly good electrical contact resistance to provide a residual-free removal of the oxides and phosphates and possible further substances without a concurrent new production of oxides or other non-conductive substances. In this context, methods in which the work piece treated has been heated substantially, have been considered as not very promising.

[0008] A chemical dissolving of layers covering the surface has the disadvantage that the corresponding processes usually are very specific and that problems occur frequently in case of a mixture or coverage of the surface layer with different other substances. Thus, different procedures, in part in several steps, must be developed for different applications.

[0009] A good possibility has been found in a mechanical removal by milling, grinding, bombarding with particles and the like. Also, these methods can be localized in a manner sufficient for electrical contacts if adequately fine tools are used. Since the layers to be removed are relatively thin, these methods are careful processes not heating the work piece too strongly.

[0010] Surprisingly, it has been found in experiments that, however, the best results could be achieved by a laser bombardment of the surface in which the substances covering the surface evaporate, are taken away by evaporation of other substances, or are taken away by the extremely rapid local heating due to thermal expansion. Therein, also other layers lying above the oxide/phosphate layers can be taken away and be removed in the same working process. Astonishingly, parameters can be found with which on the one hand the residual-free removal of the surface layers is possible, on the other hand the energy input is that small that the work piece immediately cools the respective place so that even in an oxygen containing environment, e. g. air atmosphere, substantial new oxide layers are not produced. In any case, this applies for the moment of laser bombardment as such. It is, however, important to cover the treated surface portion with a layer closing the remetallized surface again in order to avoid a slow new oxidation.

[0011] Hereto, the invention proposes a fluid substance being applied in a fluid condition in order to adapt to the surface contour to enable a complete contact and a corresponding protection. A substance can be chosen that hardens after application or at least becomes more viscous. At least in the final, i. e. in case of a hardening substance hardened condition, the substance itself must show a metallic conductivity since it takes the role of the metallically conductive surface instead of the original alloy surface.

[0012] In conclusion, the method according to the invention produces astonishingly good contact resistance values of e. g. 0.5-2.5 mΩ/mm² being completely sufficient for practical applications. The laser method has the advantage to be able to run along even complicated surface forms or tracks in short term. Also surfaces of a complicated three dimensional shape can be treated adequately. Especially it has been found that due to the short working time and the thus high throughput in case of large numbers, much more cost effective working processes can be achieved as with mechanically removing methods.

[0013] Therein, it has also been surprising that the surface portions for the electrical contacts can be covered in reasonable time periods with laser technology, being specially developed for microscopic processes, by arranging single shots side by side or by using a continuous laser bombardment, namely because of the relatively small necessary energy per surface unit, as explained above.

[0014] As said above, the invention is directed to technical oxide layers and/or phosphate layers on Al or Mg alloys. Especially anodized surfaces are relevant.

[0015] Thus, the invention relates e. g. to the removal of anodic oxidation layers on Al alloys. A special feature of the invention is that also alloys with a relevant Mg ratio can be remetallized. Thus, the invention preferably relates to alloys having a Mg ratio of at least 2 weight %, preferably at least 10, 30 or even 40 weight % Mg. These are especially the alloys AZ91 and AM50 having each around 90 weight % Mg.

[0016] As a metallically conductive flowable substance, preferably hardening materials on synthetic resin basis are considered that comprise metallically conductive particles, i. e. that are not conductive in their matrix. These can be electrically conductive adhesives, e. g. silicone adhesives. The metallic particles in the fluid material can consist of Ag or Cu. A preferred choice is silicone adhesive comprising Ag covered Cu particles. The Ag coverage provides for a very good electrical conductivity wherein the high costs of adhesives having a substantial Ag ratio can be avoided.

[0017] A special aspect of the invention is an additional application of a lacquer layer on the surface before laser bombardment, possibly having interposed additional layers. It has been assumed that especially with organic materials and lacquers crack and polymerisation processes due to the thermal energy input of the laser, which can lead to extremely resistant insulating covers, can not be avoided. Actually, however, very good results with low contact resistances can be realised.

[0018] On the other hand, the lacquer layer has the primary advantage to stabilise the layers during laser bombardment and to avoid or at least to cover cracks in the layers at the borders of the remetallized portions which are unavoidable in some cases. Therewith, together with the sealing by means of the flowable material, it can be achieved that, finally, the Al or Mg alloys can be encapsulated completely.

[0019] The term “lacquer” comprises each material having a sufficient adhesion on the surface, and being somewhat elastic in the applied condition, and which fulfils the above described function. Especially appropriate are water lacquers, especially water lacquers on acrylic resin basis.

[0020] However, also cathodic immersion lacquers (KTL lacquers) or electrophoretic lacquer layers are possible.

[0021] Such lacquer layers can be applied directly and two-dimensionally on the surface. In areas outside of the border region of the remetallized portion, in which they are principally not necessary according to the invention, they provide for an additional protective layer for the alloy or are useful in a colorization of the surface. Thus, technical limitations in a colorization of layers on Al and Mg alloys can be bypassed by using an adequately coloured lacquer.

[0022] According to another aspect of the invention, the surface portions to be remetallized are bombarded with the laser twice or multiple times successively wherein some time passes by there between, during which e. g. other surface portions can be bombarded. Thus, the energy input per shot can be limited and a too large heating can be avoided while, on the other hand, a complete removal of the layers is possible. Preferably, the surface portions to be remetallized are bombarded twice.

[0023] A special advantage of the invention is that, despite the energy input by the laser, it is not necessary (however also not excluded) to work in an inert gas atmosphere. A laser device can be used in normal room air atmosphere or in another working environment under presence of oxidizing substances. Depending on the oxidizing tendency of the environment, the maximum time period until the coverage with the fluid conductive substance is to be chosen. In air atmosphere, some hours or even days can pass between the laser bombardment and the succeeding coverage depending on the environmental temperature and air humidity.

[0024] Many different laser types can be used. Proven as useful are infrared lasers, especially Nd activated yttrium-aluminium-garnet lasers (Nd: YAG). Typical power densities of the laser bombardment are in the region of 8 W/mm² up to 20 W/mm², preferably between 8 W/mm² and 40 W/mm².

[0025] E. g. the laser can be a commercial Nd: YAG laser of the type Baasel “Star Mark”. Typical conveying speeds are between 100 and 500 mm/s, preferably between 200 and 400 mm/s. Therein, repetition frequencies of the flash lamp and thus of the laser pulse of 1-5 kHz, preferably 2-4 kHz, can be reached. A flash lamp current of 10-30 A with 380 V can be adequate.

[0026] An especially important application field of the invention is in casings and casing parts of electric or electronic apparatus which are produced increasingly from Al and also from Mg alloys for weight reasons, recently. Such casings must be electrically contacted or electrically connected to other parts due to a shielding of electromagnetic interference radiation and/or for grounding due to safety aspects. Hereto, also this conductive substance, e. g. the above mentioned silicone adhesive with Ag coated Cu particles, can be used as an adhesive or sealing. Such problems occur especially for mobile electronic apparatus because for weight reasons the alloys treated here are very frequently used here. The most important example are mobile phones.

[0027] A preferred embodiment is a die-casting part of the Mg alloy AZ91 shown in the enclosed figure. Shown is a chassis part 1 of a mobile phone casing. This chassis part 1 shall be glued to other metallic or metallically coated casing parts along lines 2 shown in the figure. It is essential on the one hand that the Mg die-casting chassis part 1 has a good surface stability and a high-grade appearance. Due to frequent hand contacts and the thus given concurrent influence of salts, weak acids and humidity as well as due to climate influences and other circumstances during a use for years, the outer surface can look improper in case of an insufficient coating. The inner surface, however, can produce particles in case of corrosion and thus cause defects of electronic parts.

[0028] It is further important for the glueing that the parts glued are connected in an electrically well conductive manner in order to produce an electromagnetic shielding of the mobile phone. In conclusion, a stable coating of Mg die-casting part 1 must concurrently enable a good electrical conductivity of the surface portions 2 used for glueing. This applies also for surface parts of contact domes 3 for a circuit board of a mobile phone which also become conductive because of the necessary grounding. Further details of chassis part 1 are not necessary for understanding the invention.

[0029] Chassis part 1 is coated with a commercial “Anomag” layer, namely an anodic oxidation/phosphatizing layer of about 3-5 μm thickness which is commercially offered by Magnesium Technology Licensing Ltd. (Auckland, New Zealand) and their contract partners. To be precise, this is a phosphate layer with incorporated oxides on the Mg alloy substrate.

[0030] The Anomag layer is covered with a acrylic water lacquer before laser bombardment in order to encapsulate the surface and improve their appearance. The laser bombardment is done in normal atmosphere and room temperature.

[0031] The surface is thereafter treated with a commercial Nd: YAG laser at chassis 2 and contact domes 3 as shown. This laser is Q-switched. Lines 2 and areas 3 shown in the figure are tracked twice, wherein one shot is set beside the other, to be precise. The distance, size, and energy per shot can be found empirically in order to provide for a continuous track having a sufficient width. The track width should be not too small in order to optimize the electrical contact resistance to the other casing part. On the other hand, the track width should be not too large and should be covered by the later applied adhesive bead completely. Here, it is about 1 mm. Therein, the energy input per shot should not be unnecessarily high in order to avoid a substantial heating in larger depths. Due to the double bombardment, the energy per shot can be decreased further. Here, 20 W/mm² are used per shot.

[0032] The laser used is a “Baasel StarMark” set to a pulse repetition frequency of 3 kHz. The conveying speed is about 300 mm/s and thus enables a very rapid work.

[0033] After metallizing, the metallized portions are brush-coated with a silicone adhesive with Ag coated Cu particles. This should be done as soon as possible after the laser bombardment in order to optimize the contact resistance. This adhesive can also be used for sealing and connecting during glueing the chassis part shown with other casing parts of the mobile phone.

[0034] Such other casing parts are also metallic or metallically coated and are glued such that an electric contact to the adhesive is given. In this way, a sealed and electrically shielded casing can be produced, finally. 

1. A method for producing a metallically conductive surface portion (2, 3) in a surface (1) of an alloy being oxidized and/or phosphatized by anodisation, said alloy comprising Al and/or Mg, in which method said surface portion (2, 3) is bombarded with a laser in order to remove the oxide/phosphate layer and said surface portion is covered thereafter with a metallically conductive fluid substance being so fluid during application on said surface portion that it can well adapt to the surface contour of said surface portion.
 2. (Cancelled)
 3. (Cancelled)
 4. A method according to claim 1, in which said surface (1) comprises both oxides and phosphates.
 5. A method according to claim 1, in which said alloy comprises at least 2% Mg.
 6. A method according to claim 5, in which said alloy is AZ 91 or AM
 50. 7. A method according to claim 1, in which said fluid substance is a hardening substance on synthetic resin basis comprising metallically conductive particles.
 8. A method according to claim 7, in which said hardening substance is a silicone adhesive.
 9. A method according to claim 7, in which said metallically conductive particles are silver particles or silver-covered metal particles.
 10. A method according to claim 1, in which at least a part of said surface (1) corresponding to the border of said surface portion (2, 3) is covered with a lacquer before said laser bombardment.
 11. A method according to claim 10, in which said lacquer is a water lacquer.
 12. A method according to claim 11, in which said lacquer has an acrylic resin basis.
 13. A method according to claim 10, in which said lacquer is applied electrophoretically.
 14. A method according to claim 10, in which said applied lacquer layer is coloured or is treated with a colorization process.
 15. A method according to claim 1, in which said surface portion (2, 3) is successively bombarded at least twice.
 16. A method according to claim 1, in which said laser bombardment is performed in air atmosphere.
 17. A method according to claim 1, in which a Q-switched Nd: YAG laser is used for said laser bombardment.
 18. A method according to claim 1, in which a power density on said surface (1) in the region from 8 W/mm² up to 70 W/mM² is used for said laser bombardment.
 19. A method according to claim 1, in which said surface is the surface or a surface portion (1) of a casing of an electric or electronic device, especially a mobile phone.
 20. A method according to claim 19, in which a plurality of casing parts is connected and/or sealed against each other in an electrically conductive manner with that metallically conductive substance. 